1. Field of the Invention
The present invention relates to a twist drill for drilling holes in a material. Twist drills, in general, have drill points or tips that initially cut into a material to be drilled. The material is then subsequently contacted by the cutting edges and cutting faces located adjacent the drill point or tip. The cutting edges produce chips which are deflected and shaped by the drill flutes. The drill flutes can extend over a substantial length of the body of the drill in a helical manner. The drill body is held in a drill chuck, which holds the drill body securely.
It is important to note that substantial stresses are exerted on a drill during drilling. Therefore, the drill should preferably be sufficiently strong and durable. Further, the stresses exerted on the drill tip, cutting faces, and cutting edges are higher than those stresses exerted on the drill body. Therefore, it is preferable that the drill tip, cutting faces, and cutting edges be stronger and more durable than the drill body.
In addition, because of the substantial stresses exerted on the drill tip, cutting faces, and cutting edges, it usually becomes necessary to sharpen these areas of the drill by grinding them. However, such a sharpening process over time shortens the length of the drill. Also, the sharpening process requires that the entire drill body be sufficiently strong and durable such that as the drill is ground and sharpened, the cutting end of the drill is not weakened and thus rendered inadequate to withstand the high stresses exerted on the cutting end.
To overcome the disadvantages of sharpening the drill tip, cutting faces, and cutting edges, it is well known to produce a replaceable insert that can be inserted into the body of the drill. The insert, when sufficiently worn by use in drilling, can be removed and replaced with a new insert. The insert often contains the entire cutting end of the drill, including the drill tip, the cutting faces, the cutting edges, as well as portions of the chip flutes. Such an insert is connected to the drill body in a secure but removable fashion. Other inserts can be less comprehensive, and may only contain portions of the cutting end, such as the drill tip and/or portions of the cutting edges and cutting faces.
This invention further relates to a rotary cutting tool, e.g. a drill, a milling cutter, a reamer or similar device with a tool head made of or including at least one hard cutting material, e.g. a tungsten carbide cutting alloy, oxide ceramic or similar material and with a tool shank that fixes the tool head in position coaxially, which tool shank, on its holding end opposite the machine-side chucking end, frictionally presses a fixing lug that projects coaxially out of the shank-side end surface of the tool head, whereby the end surface functions as an axial stop toward the tool shank, with the inner flanks of two clamping extensions that project essentially in the axial direction from the tool shank and are one piece with the tool shank.
Solid tungsten carbide drills (VHM drills) of the prior art are regularly manufactured in one piece. On these drills, the drill tip and drill shank are. inseparable. However, the prior art also includes drills in which a VHM cutting body is soldered into the shaft or in which the VHM cutting body is detachably connected with a steel shank. On a drill of the prior art described in WO 96/11079 the one-piece tool head is clamped between the flanks of two clamping extensions that project in the axial direction from a tool shank which is made of high-speed tool steel. This combination drill with a steel shank and replaceable carbide tips can be manufactured easily only for drill diameters that are larger than 10 mm. A particular problem with such drills is that it is difficult to achieve a degree of precision and accuracy sufficient to guarantee that the tips can be replaced without problems and that the parts are absolutely concentric.
2. Background Information
On a drill of the prior art described in EP-B1-0 118 806 having a tool head made of or including at least one hard cutting material, e.g. a tungsten carbide cutting alloy, oxide ceramic or similar material and with a tool shank that fixes the tool head in position coaxially, which tool shank, on its holding end opposite the machine-side chucking end, frictionally presses a fixing lug that projects coaxially out of the shank-side end surface of the tool head, whereby the end surface functions as an axial stop toward the tool shank, with the inner flanks of two clamping extensions that project essentially in the axial direction from the tool shank and are one piece with the tool shank, the clamping of the tool head is accomplished by means of a fastening peg that is attached to the drill head and is concentric to the drill axis, and which holds a cutting insert that forms the tool head concentrically in a peg receptacle on the tool shank. The fastening peg is undercut in the periphery of the peg so that it can be inserted from the chip flute laterally into the peg receptacle. After the insertion into the peg receptacle, the cutting insert and the fastening peg that is one piece with it are twisted in the manner of a bayonet connection opposite to the direction of rotation of the drill until stop surfaces provided on the fastening peg come into contact with corresponding stop surfaces of the peg receptacle. The desired frictional clamping of the cutting insert impossible because the cylindrical surface of the fastening peg has undercut stop surfaces on the periphery of the peg. The cutting insert is braced against the cutting forces by the rear wall and the shank-side edge of the recess that is designed to hold it. The cutting forces press the cutting insert into its seat so that it is fixed in position without additional fastening means. This arrangement means that the essential replacement of the fastening screw that holds the cutting insert is a complicated and time-consuming operation. The undercut on the periphery of the peg also has an adverse effect on the desired accuracy of rotation.
U.S. Pat. No. 2,294,969 A describes a drill that is equipped in the vicinity of its cutting tip with tungsten carbide cutting plates, and which is equipped on its cutting end with an axially projecting tip drill insert. The purpose of this axially projecting tip drill insert is to protect the tungsten carbide cutting plates. The middle part of the tip drill insert, which middle part is surrounded over its entire periphery by the drill shank, is realized in the form of a clamping cone. Moreover, the end of the tip drill insert farther from the drill tip is square, to ensure a torque drive by the drill shank. The drill tip area of the tip drill insert that has the chip flutes on its generated surface is surrounded by axial extensions of the drill shank. However, these extensions do not perform any clamping action.